The problems associated with providing efficient signal propagation at microwave frequencies, that is at frequencies generally above about 100 MHz and more particularly above about 1 GHz, are substantially different than the problems encountered at lower frequencies. The use of ordinary wires is generally impractical at microwave frequencies. Although coaxial cables may be used for some connections, they are generally not suitable for interconnections between the comparatively small components or electronic elements that frequently make up microwave circuits. Hence it has become common in the art to use dielectric circuit boards with thin flat conductors layers bonded to one or both sides. The conductor layers are etched or otherwise shaped to provide a variety of substantially planar conductive traces that interconnect the various passive and/or active components or elements mounted on the board. These conductive traces also permit input and output connections to the circuit board.
For efficient propagation of microwave signals it is important that the conductors on the circuit board form a transmission line having a known and well controlled characteristic impedance. Two well known conductor arrangements in widespread use for microwave circuits are illustrated in FIGS. 1A-B and 2A-B. FIGS. 1A-B illustrate in plan and cross-sectional view respectively, microstrip line 10 comprising dielectric substrate 12, front surface conductor 14 and rear surface ground plane 16. The characteristic impedance of this microwave conductor arrangement is determined primarily by thickness 13 and the dielectric constant of substrate 12 and by width 15 of surface conductor 14.
A coplanar waveguide arrangement suitable for use on microwave circuit boards is shown in FIGS. 2A-B. Coplanar transmission line 20 comprises dielectric substrate 22 of thickness 23 having conductors 24, 24', 28 on a principal surface thereof. Conductors 24, 24' are ground or reference conductors and conductor 28 is the active (un-grounded) conductor. Reference conductors 24, 24' and active conductor 28 are separated by gaps 25, 25' of widths 26, 26'. Active conductor 28 has width 29. The characteristic impedance of coplanar waveguide 20 depends primarily on the width of active conductor 28 and the widths 26, 26' of gaps 25, 25'. Gaps widths 26, 26' on each side of conductor 28 need not be equal, but that is convenient.
FIG. 3 shows in plan view, illustrative coplanar microwave circuit board 30 containing microwave component 31, as for example, a microwave transistor or monolithic microwave integrated circuit (MMIC). Circuit board 30 comprises dielectric substrate 32 on which are coplanar ground or reference conductors 34, 34', 34", and active conductors 38, 38' and 38" separated from the reference conductors by gap 35, 35', 35". Input-output connections are provided by, for example, wire-bonds or other leads 36, 36', 36" and active device 31 is coupled to conductors 38, 38', 38" by, for example, further wirebonds or leads 37, 37', 37". Those of skill in the art will understand, that FIG. 3 is intended merely to illustrate an exemplary arrangement for a microwave circuit board having at least one component and not to be limiting. Many other arrangements are possible and frequently microwave circuit substrates or boards will have large numbers of components thereon. For convenience of explanation, further reference to 34, 35, 36, 37, 38, 40 and 42 unless specifically noted, are intended to include reference to the corresponding primed numbers as well. Dashed line 4A in FIG. 3 indicates that portion of circuit board 30 which is shown somewhat enlarged in FIGS. 4A-B.
Because I/O connections 36 must generally be robust and easily applied, external lead bonding region 40 of leads 38 must have dimensions larger than is otherwise needed elsewhere in circuit 30. The lateral dimensions, e.g., the width of bonding regions 40 and gaps 35 associated therewith are generally chosen so as to provide a characteristic impedance of the coplanar waveguide at the bonding location that is well matched to the external connection. Fifty ohms is a typical target value for the characteristic impedance of the coplanar waveguide formed by bonding regions 40.
Since component 31 is frequently very small, associated bonding portions 42 of conductors 38 where leads 37 attach are generally substantially smaller, i.e., narrower, than external bonding regions 40. For reasons having to do with manufacturing tolerances, gap 35 are generally not scaled with the reduction in lead widths. Hence, as leads 38 become narrower at or near bonding regions 42, their characteristic impedance rises. This produces a significant impedance mismatch problem which makes it difficult to efficiently couple energy into and out of component 31. It is not always possible to modify the line dimensions and/or substrate dielectric constant in such a way to eliminate the impedance mismatch.
There have been attempts in the prior art to deal with impedance mismatch in circuit board transition regions. For example, U.S. Pat. No. 4,906,953 to Li et al., describes an impedance matching arrangement using sloped surfaces to provide an interconnection between microstrip and coplanar waveguide without use of via holes. However, production of such sloped surfaces is comparatively expensive and does not suit all situations.
Other arrangements, well known in the art have also been tried with varying degrees of success. However, many of these involve undesirable trade-offs with respect to manufacturability, yield, cost and/or reliability. Hence, despite many years devoted to microwave circuit development, impedance mismatch problems still remain, especially in connection with efficient coupling to MMIC's and the like.